Edge radial electric fields were induced in the edge of the TEXTOR tokamak by means of a polarization electrode in order to study their influence on the plasma edge profiles and its confinement. The studies include the generation of H-mode behaviour with either positive or negative polarity. Particle confinement ( T~) of deuterium and of impurity ions as well as energy confinement (73 are investigated. For positive fields which remain below the threshold for the L-H transition, an interesting regime of reduced particle confinement without noticeable energy confinement loss is found. A strong asymmetry in the edge density profiles with respect to the electric field sign is observed at these low polarization voltages. Above the threshold, H-mode behaviour with increased energy confinement and especially particle confinement can be produced with either polarity of the applied electric field. It is, however, found that, whereas the energy confinement in positive H-modes is at least as good as that in negative ones, the ratio T~/ T~ is about three times lower in the former case.
H mode behaviour is usually linked to the existence of radial electric fields or to their shear at the edge of tokamaks. The mechanisms are investigated by which such fields are induced in the plasma edge and by which the profile shaping is obtained when radial currents are imposed by electrode polarization. Earlier detailed experimental field measurements are successfully compared with a theoretical conductivity model in which neoclassical non-ambipolar transport and mobility through ion-neutral collisions are predominant. Strong neoclassical viscosity in the bulk plasma allows significant fields to develop only at the very edge of the plasma. There, a delicate balance between viscosity and ion-neutral friction takes place, which strongly affects the magnitude of the fields and the spatial location and the threshold condition for L-H field bifurcation. It is also shown how to verify experimentally the neoclassical diffusion coefficients in the plateau regime
An ovcrvicw is prcscnlcd of rccent results on the crealion of elcctric ficlds in thc edge of limilcr or divcrlor tokamaks. The practical implcmcntalion and thcorctical basis of scvcral schemes, gencrating radial and I or poloidal ficlds. arc outlincd. Thc manipulation of cdge and scrapc-off-layer profiles and the conlrol 01 pdniclc cxhaust i s discussed. Contributions of biasing experiments to H-modc physics arc highlightcd. Somc prospects for biasing in next gcncration tokamaks arc linally givcn.
The 11th European Conference on Controlled Fusion and Plasma Physics was held from 5 to 9 September 1983 in the Kármán Auditorium of the University of Aachen, FRG. The Conference, organized by the Nuclear Research Centre Jülich, was attended by nearly 500 participants from over 30 countries, among them guest delegations from several non-European countries. The scientific material was presented in 24 invited papers, 36 short oral contributions, and about 250 posters spread over five poster sessions.The present review is necessarily limited and cannot give proper credit to each work presented at the Conference. According to the Conference programme, this report is divided into ten sections with emphasis on magnetic confinement and related problems and topics.
We show that it is possible to measure the parallel and the perpendicular flow in a strongly magnetized plasma, using a probe with two current collecting surfaces, inclined with respect to the magnetic field. The method is based on a one-dimensional fluid model, which we use to relate the unperturbed velocity components (the quantities we wish to measure) to the velocity and density perturbations caused by the probe. In this way we are able to establish a link between the electric saturation currents collected by the probe surfaces and the unperturbed velocity. The role of the cross-field transport into a pre-sheath of a collecting probe surface and the boundary condition for the flow velocity at the magnetic pre-sheath entrance are also investigated. § Partner in the Trilateral Euregio Cluster (TEC).
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